Frequently Asked Questions

The Exar controllers, regulators and power modules are powered by an internal LDO to produce the Vcc rail that their circuits need to run properly. If the input voltage that feeds the LDO is the minimum Vin as specified on the datasheet or above, the Vcc is enough to power the device and the Vin, Vcc and PVIN pins can be connected together.

If the input voltage is 5V, then the Vin and Vcc pins can be tied together and the input voltage is fed to the PVIN pin.

If input voltage is below 5V, then a separate low current 5V supply (common in many systems) is connected to the Vin and Vcc pins which are tied together to keep the device running properly. The input voltage is connected to the PVIN pin.

Voltage mode PWM

Voltage mode PWM is a simple technique that uses a single loop to control the output voltage. As shown in Figure 1, the output voltage is compared to a reference voltage with an error amplifier. The output of the error amplifier is then compared to a sawtooth and that output is used to drive the MOSFET, usually via a voltage divider.

 


Figure 1

 

As shown in Figure 2, the output voltage is modulated by turning the high-side FET on (on-time) with the pulse width and turning the low side FET off. At the end of the pulse, the high-side FET is turned off (off-time) and the low side FET is turned on until the next pulse. Vout = On-Time/Period * Vin.

 

Figure 2
 

The advantages of voltage mode PWM is that it is a very simple, common, smaller solution with good accuracy. The disadvantages are that complex frequency compensation is required (two poles) to stabilize the loop and because trailing edge control is most commonly used, there is a delay in load step response.

Current mode PWM

With voltage mode PWM, current is less known. For better control, current mode PWM senses the inductor current and it is compared to the reference voltage as shown in Figure 3.

 


Figure 3

 

Although the current has to be sensed with accuracy and introduces noise, the advantages of current mode PWM are easier loop compensation (less compensation needed with one pole), and it is easier to implement over-current protection and parallel currents to the output.

Standard Constant On-Time (COT)

As opposed to PWM, the pulse width in COT is always the same as shown in Figure 4. Instead the off-time length varies (as does the frequency) which modulates the output. As the Vout increases, the off-time of the duty cycle increases (frequency decreases) and the fixed on-time produces a lower duty cycle. This transfers less energy to the output and lowers the Vout. More simply said, as Vout increases, the duty cycle decreases. Conversely, as the Vout decreases, the off-time of the duty cycle decreases (frequency increases) and the fixed on-time produces a higher duty cycle. This transfers more energy to the output and raises the Vout.

 


Figure 4

 

The advantages of standard COT are very fast transient response, simplicity (inexpensive) and that frequency compensation is not complex as it is in PWM control. However, the feedback signal tends to have low amplitude and signal to noise ratio, making it very noise sensitive. Also, the output voltage is higher than the reference voltage and the ripple is dependent on and sensitive to the output capacitor ESR. This introduces a DC offset which is the average amount the output voltage is over the reference voltage. It is also jitter prone and the frequency changes during the load steps.

Some solutions solve the noise sensitivity by having one of two options that condition the feedback signal but introduce delays. One tradeoff provides faster transient responses; the other allows low ESR output capacitors to be used.

MaxLinear’s patented COT

MaxLinear’s patented COT architecture however conditions the reference instead as shown in Figure 5. The MaxLinear devices create their own emulated ramp that is insensitive to noise and the ESR of the capacitor. Since the output capacitor ESR does not affect it, low ESR ceramic capacitors can be used and maintain stability without decreasing speed. In addition, the Vout and reference voltage are compared and that result controls the ramp circuit. This creates a slower loop where the output voltage is averaged out and the DC offset is not introduced as in standard COT.

 


Figure 5

 

MaxLinear’s COT still has the standard COT advantages of very fast transient response, simplicity and no complex frequency compensation in addition to not having DC offset or ESR value sensitivity. MaxLinear’s COT architecture provides exceptional line and load regulation.

The XR76117 and XR76121 are identical to each other except for their output current rating. Similarly the IR3824/IR3825/IR3829 are identical to each other except for the differences shown in this section. Output current ratings are summarized in Table 1.

  

Spec

Exar XR76117 / XR76121

Infineon IR3824 / IR3825 / IR3829

Max IOUT

15A (XR76117); 20A (XR76121)

15A (IR3824); 16A (IR3829); 20A (IR3825)

VIN range

4.5V to 22V

5V to 21V

VOUT range

0.6V to 18V

0.6V to 0.86 x VIN

Frequency range

200kHz to 1MHz

Up to 1.5MHz

Temperature range

-40°C to 125°C

-40°C to 125°C

Supply current

17mA

20mA


Table 1: Major Specification Comparison 

 
The XR76117 / XR76121 can be soldered into a IR3824/25/29 socket, only minor board stuff options are required. The following and Application Note ANP-49 discuss how both series can occupy the same socket on a PCB.

 

Pin-out comparison

 

 
                                                                                 
Figure 1: Pin-out Comparison 
 

Pin

XR76117 / XR76121

IR3824 / IR3825 / IR3829

Same

Function /

Connection?

Changes Required to Drop XR76117/21 into IR3824/25/29 Socket

1

FB

Fb

Yes/Yes

None

2

FCCM

NC

No/No

Must either be tied to VCC for FCCM operation or tied to GND for CCM/DCM operation. Pull-up or pull-down resistors may also be used. See Figure 2 and XR76117 or XR761121 datasheet for more information.

3

AGND

Comp

No/No

Add a 0Ω resistor to jumper pin 3 to pin 4 and do not stuff compensation Cs/Rs. Refer to Figure 2.

4

AGND

Gnd

Yes/Yes

None

5

TON

Rt

No/Yes

In both cases, this pin sets frequency and requires a resistor to GND. However, a different resistor value must be used for the XR solution. Refer to the XR76117 / XR76121 datasheet for resistor value calculation.

6

ILIM

ILIM

Yes/No

Add a 0Ω resistor to connect to SW. Do not stuff resistors to VCC or GND. Refer to Figure 2.

7

PGOOD

PGood

Yes/Yes

None

8

VSNS

Vsns

Yes/Yes

None

9

VIN

Vin

Yes/Yes

None

10

VCC

Vcc/

LDO_Out

Yes/Yes

None

11

PGND

PGnd

Yes/Yes

None

12

SW

SW

Yes/Yes

None

13

PVIN

PVin

Yes/Yes

None

14

BST

Boot

Yes/Yes

None

15

EN

Enable

Yes/Yes

None

16

SS

NC

No/No

Add an external capacitor to GND. See Figure 2 or the XR76117/XR76121 datasheet for more information.

17

AGND PAD

GND

Yes/Yes

None


Table 2: Pin-out Comparison & Changes Required to Drop XR76117 / XR76121 into IR3824 / IR3825 / IR3829 Socket 

 
Board Stuff Option Schematic
 

The PCB board can be easily designed to drop-in the XR76117 or XR76121 while maintaining compatibility to the IR3824/25/29. In Figure 2 below, pinning for both series are represented. The Exar XR76117 and XR76121 pin names do not have parenthesis, and the same corresponding pins for the IR3824/25/29 are in parenthesis. As shown in the legend, the components in green boxes are added and the ones in red boxes are omitted when using the XR76117 or XR76121. Both a pull-up and pull-down are shown for pins 2 and 6, but only one or the other will be present depending on the application. So, 4 passives (red) will not be populated while 3 passives and a jumper will be populated.

 
 


Figure 2: Addition and Omission of External Components 
 

With a plethora of equipment with built in USB ports, USB hubs assist by expanding the number of USB ports available to plug devices into in a network. In its simplest form, a USB hub is plugged into a host computer’s USB interface. A hub has one upstream path (going back towards the host’s USB interface) and multiple downstream paths (going towards the end devices). Another downstream hub could be plugged and cascaded into the first hub and so on up to 7 tiers and 127 ports. There are limitations on USB cable length, however a USB hub can function as a repeater if more
length is needed.

 

USB is governed by industry specification http://www.usb.org/developers/docs/usb20_docs/ .

 

MaxLinear XR22404 and XR22414 USB hubs have 4 available downstream ports while the XR22417 provides 7. All 3 parts have a USB 2.0 compliant interface, meaning that the upstream is capable of high speed (480Mbps) and may operate at high (480Mbps) or full speed (12Mbps). Downstream can operate and high (480Mbps), full (12Mbps) or low speed (1.5Mbps).

 
 

A bus-powered hub is powered from upstream, whereas a self-powered hub has its own power supply. Governed by industry specification http://www.usb.org/developers/docs/usb20_docs/, a port has a maximum of 500mA which powers itself and up to 4 downstream ports up to 100mA each.XR22404, XR22414 and XR22417 all support self-powered while XR22404 and XR22414 also can support bus-powered operation. The XR22417 is self-powered only so that it can support 7 downstream ports. Bus-powered hubs provide 5V power up to 100 mA for low powered devices and up to 500mA for high powered devices and hubs. XR22404, XR22414 and XR22417 each have internal 5V to 3.3V and 3.3V to 1.8V regulators and do not require external LDOs. In fact, the XR22414 (if powered by 5V) can provide 3.3V power to other external circuitry.

 

Hubs that provide downstream battery charging such as the XR22404 must be in self-powered mode to have enough power. BC1.1/1.2 compliant hubs can have downstream ports that are either SDP (Standard Downstream Port) that do not allow charging, DCP (Dedicated Charging Port) that are only used for charging or CDP (Charging Downstream Port) that can be used for charging or as a standard port. SDP, DCP or CDP is determined by the device attached.

 A transaction translator (TT) segregates and translates between high speed (USB 2.0 / 480Mbps) upstream ports and USB 1.1 downstream ports. USB 1.1 can be low speed (1.5Mbps) or full speed (12Mbps). USB 2.0 devices operate at full or high speed, and compliant USB 2.0 hubs have high speed capable upstream ports like the XR22404, XR22414 and XR22417. Downstream ports may be high, full or low speed. When a USB 1.1 device is plugged into a USB 2.0 hub, the TT recognizes this and translates USB 1.1 to USB 2.0 upstream.
 

STT (Single Transaction Translator) is where one TT splits transactions and polls round robin to translate to multiple downstream peripheral devices, such as in the case of the XR22404 that shares the bandwidth. MTT (Multiple Transaction Translator) is where several are provided. The XR22414 provides one for each of the 4 downstream ports, while the XR22417 provides one for each of the 7 downstream ports. A dedicated TT for the downstream ports provides each the highest bandwidth capability, 12Mbps each in the case of full speed.

 

  
 
 

Either an individual or ganged power mode can be employed. In ganged mode all ganged ports are monitored by one power monitoring device and global current sensing is used. However in an over-current condition, all ganged ports are disabled. In individual mode each downstream port monitors over-current and can disable power independently. XR22404 uses ganged power mode and global overcurrent sensing. XR22414 can be configured for either ganged or individual power mode as shown in its datasheet. XR22417 uses individual power mode.

 

The hub then signals the USB host and the host marks the port. SP2525A or SP2526A can be used in conjunction with the XR22404, XR22414 and XR22417 devices.

For some UARTs, Microsoft certified drivers are available for Windows Operating System and can be downloaded via Windows Update. These drivers and others, including for Linux and other Operating Systems can be found by visiting https://www.exar.com/design-tools/software-drivers Please note Software Driver Use Terms.

 

 
You can also get to this link by going to the exar.com website, clicking on Support (in black bar near top of page), then click on Design Tools, then under Evaluation Hardware and Software (towards right of page) click on Software Drivers.
 
 

Click on the version link under Driver Version of the desired type of UART, part number and operating system. A zip file is downloaded which contains a ReadMe file with instructions.

Links to datasheets and product family pages are in the software driver table for easy reference. 

For RS-232 it is 50 feet (15 meters), or the cable length equal to a capacitance of 2500 pF, at a maximum transmission rate of 19.2kbps. When we reduce the baud rate, it allows for longer cable length. For Example:

 

Baud Rate (bps)

Maximum RS-232 Cable Length (ft)

19200

50

9600

500

4800

1000

2400

3000

 
For RS-485 / RS-422 the data rate can exceed 10Mbps depending on the cable length. A cable length of 15 meters (50 feet) will do a maximum of 10Mbps. A cable length of 1200 meters (4000 feet) will do a maximum of 90kbps over 24 AWG gauge twisted pair cable (with 10 pF/ft). Refer to Annex A TIA/EIA-422-B.